Travelling wave tubes



June 9, 1959 V D. 0. ROGERS p TRAVELLING WAVE TUBES 2 Sheets-Sheet 1 Filed Feb. 2, 1954 v lnenlor D. C. ROGERS A Horn e y D. C. ROGERS TRAVELLING WAVE TUBES June 9, 1959 2 Sheets-Sheet 2 Filed Feb. 2. 1954 Inventor D. C- ROGEBS Attorney TRAVELLING WAVE TUBES Douglas Cecil Rogers, London, England, assignor to International Standard Electric Corporation, New York, N.Y., a corporation of Delaware Application February 2, 1954, Serial No. 407,732

' Claims priority, application Great Britain March 26, 1953 4 Claims. (Cl. 315-35) beam. One of the forms of slow wave structure mostcommonly in use to-day is a conductor formed as a helix surrounding the electron beam and contained within the evacuatedenvelope of the travelling wave tube. It is common practice to pass the travelling wave tube through a section of hollow rectangular wave guide, so as to position an end or the helix just within the wave guide, a lhigh'fr equency choke being provided opposite the said end of the helix by means of a choke member inside the tube envelope which is coaxial with an outer choke member continuous with the wave guide wall. The end of the helix is joined to the inner choke member by a straight rod which behaves as a probe antenna, one end of which is efiectively at earth potential by virtue of the capacitative coupling between the inner and outer choke members, the other end being in electrical and mechanical contact with the helix. The system is somewhat analogous to that of an earthed quarter-wave antenna.

With the known arrangement mentioned above, the electron beam must travel the length of the antenna before entering the helix and, therefore, there is a noneifective region of the axial length of the electron beam on which interaction with the electromagnetic waves does not occur. Not only does this non-eifective region of the electron beam result in an increased length of the travelling-wave tube, which is undesirable purely from the aspect of physical size, but it increases the difiiculty of maintaining the electron beam in focus over the length of the tube in which interaction takes place.

It is an object of the present invention to provide a coupling means between a helix in a travelling-wave tube and an electromagnetic wave guidethrough which the tube projects which will reduce the length of the'region in which the electron beam and the slowly propagated electromagnetic waves do not-interact. 1

According to the present invention therefore there is provided a travelling wave tube comprising a helix, means for projecting an electron beam along the axis of the helix and means for coupling the helix to a wave guide comprising a probe member one end of which is connected to the helix whereas the other end is connected to another member in the tube which is adapted to form electrically a part of the wave guide wall, the said probe member being shaped to bring the end of the helix so close to the said other member that the helix extends through a substantial portion of the width of the wave guide.

In preferred embodiments of the invention the probe member is U-shaped. According to another aspect of the invention there is provided a travelling wave tube comprising electrode means for projecting an electron beam along an axis to an electron'collector electrode, a helix surrounding the said axis for propagating electromagnetic waves in a slow mode to intercharge energy with the said beam and co-operating, during operation of the tube, with a wave guide through which the tube projects; means for coupling the said helix as a transmission path to the said wave guide, the said means comprising a generally U shaped probe having arms parallel to the said axis, one arm being secured to the end of the helix and the end of the other arm being secured so that, during operation of the tube, it is capacitatively coupled to the wall of the said wave guide opposite the said end of the helix.

The invention will be described with reference to the accompanying drawings in which:

Fig. 1 shows, diagrammatically, a travelling-wave tube according to the present invention inserted in its cooperating apparatus;

Fig. 2 is a diagram illustrating a known method of coupling between helix and rectangular wave guide; and

Fig. 3 illustrates the coupling arrangement of the present invention.

In Fig. 1 a travelling wave tube 1 comprises an electron gunhoused within a bulb portion 2 of a tube envelope which is provided with an end cap 3 carrying connecting pins for supplying the appropriate polarising potentials to the tubeelectrodes. The base also carries a locating flange 5 for positioning the tube within the associated apparatus, but as we are not here concerned with the mounting arrangements of the tube within its associated apparatus, nor with the means for mounting the components of the travelling wave tube within the tube envelope, in order to simplify the drawings these items have not been shown in any of the figures.

At the other end of the tube a collector electrode 6 is provided, carrying a cooling member 7. Between the bulb 2 and the collector electrode 6 the envelope of the tube is of reduced diameter and surrounds a helix 8.

The travelling wave tube 1 projects through an input wave guide 9 near the electron gun end and an output wave guide 10 near the collector electrode, for coupling to the respective ends of the helix, the wave guides being provided with extensions 11 and 12, respectively, each provided with a short-circuiting plunger 13 to adjust the impedance match between the respective rectangular wave guides and the helix. To reduce leakage of electromagnetic wave energy into the gun and collector electrode portions of the travelling wave tube opposite the respective ends of the helix, high frequency chokes are provided passage of the electron beam.

' The coupling means between each of the wave guides 9 and 10 and the respective end of the helix comprises a probe antennae 16 connected between the inner choke member 15 and the adjacent end turn of the helix, and will be more fully described below.

The travelling wave tube 1 is shown surrounded by a set of solenoids 17, 18 and 19 respectively, which are energised during operation to provide the requisite axial magnetic field for guiding the electron beam between the electron gun and the collector electrode 6.

Fig. 2 illustrates diagrammatically, a common prior coupling arrangement used with travelling wave apparatus in other respects fundamentally similar to that of Fig. 1. An input or output feeder wave guide is shown in section at continuous with an extension 21 containing a short-circuiting piston 22. An outer wave guide choke member 23 projects from the rear wall 24 of the wave guide. The travelling wave tube envelope 25 projects through the wave guide section 20, 21 and encloses a helix 26 and an inner choke member 27 in the form of a hollow cylinder partially closed by an end disc 28 apertured at 29 for passage of the electron beam. Due to the capacitative coupling between members 23 and 27, the disc 28 is effectively a continuation of the rear wave guide wall 24. A rod 30 is secured to the disc 28 and projects parallel to the axis of the electron beam and helix across the wave guide 20. A few turns of the helix 26 project inside the wave 'guide 20, the end turn being secured to the end 31 of the rod 30, which rod forms a probe antenna virtually earthed at one end by the disc 28.

It will be observed that in the region between the disc 28 and the end 31 of the probe antenna, there can be no interaction between the electron beam and the slowly propagated electromagnetic waves guided by the helix. For use at frequencies in the region of 4,000 mc./s. the length of the probe antenna would commonly be of the order of 0.4 inch, which, if reproduced at both ends of the tube may amount to an appreciable fraction of the total length over which the electron beam focussing is critical. A saving of even some of this extra length results in a very worth while reduction in the size and weight of the focussing coils and associated structure of the travelling wave apparatus. This reduction is achieved in the present invention by utilising a folded construction for the antennae in place of the straight rod 30.

Referring now to Figure 3, which shows the modification of the arrangement of Figure 2 in accordance with the present invention, and in which the same reference numerals are used to denote similar components, in place of the rod 30, a U shaped probe antenna 32 is provided having unequal legs parallel to the axis of the electron beam and helix. The longer leg 33 is secured to the disc 28 and the end 34 of the shorter leg is joined to the end of the helix 26. The end of the helix must not, of course, protrude beyond the rear wall 24 of the wave guide and must certainly not foul the disc 28, for which reasons it is necessary that the arms of member 32 must be of unequal length. The difference between them may, however, be quite small and the optimum dimensions and proportioning of the probe antennae are best determined empirically.

In a typical embodiment of the invention for use at 4,000 mc./s. the wave guide 20 had transverse dimensions of 2 in. x inch and the helix consisted of 0.028 inch diameter wire, Wound at 20 turns per inch with an internal diameter of 0.131 inch. A suitable probe antenna had the base of the U, flattened at right angles to the arms, the internal separation between which was 0.125 inch. The plane of the U was substantially parallel to the helix. The overall length from the base of the U to the end of the longer leg secured to the disc 28 was 0.650 inch, the other leg, being shorter by 0.100 inch. With this coupling arrangement there was no noticeable diiference in efiiciency of coupling or matching between wave guide and helix as compared with the straight probe antenna but the non-eifective region of the electron beam was reduced at either end of the travelling wave tube by nearly one half.

It will be appreciated by those skilled in the art that, while an embodiment of the present invention has been described with reference to a travelling wave tube having a single helix coupled between input and output wave guides, the invention is equally applicable to the coupling between a rectangular wave guide and a short length of helix which can be used, for example, to launch space charge waves upon an electron beam. Again, in the embodiment described, the plane of the U of the probe antenna was substantially parallel to the axis of the helix; in other embodiments it has been convenient to have the plane of the U at right angles to the helix axis.

While the principles of the invention have been described above in connection with specific embodiments, and particular modifications thereof, it is to be clearly understood that this description is made only by way of example and not as a limitation on the scope of the invention.

What I claim is:

1. A travelling wave tube comprising electrode means for projecting an electron beam along an axis to an electron collector electrode, a helix surrounding the said axis for propagating electromagnetic waves in a slow mode to intercharge energy with the said beam and cooperating, during operation of the tube, with a wave guide through which the tube projects; means for coupling the said helix as a transmission path to the said wave guide, the said means comprising a hollow cylindrical element of substantially the same outer diameter as the diameter of said helix capacitively coupled to the wall of the said wave guide opposite the said end of the helix, a generally U shaped probe having arms parallel to the said axis, one arm being secured to the end of the helix and extending along said helix away from said end and the end of the other arm being secured to said cylindrical element.

2. A tube according to claim 1 in which said cylin der element comprises an inner wave guide choke member further comprising an outer choke member surrounding the said tube and projecting from the rear of the said wave guide opposite the said end of the helix.

3. A tube according to claim 2 in which the said choke member comprises a disc, apertured for passage of the electron beam, closing the front end of the said choke member, the said arm of the probe being secured to the said disc.

4. A tube according to claim 3, in which the base of the U formed by the said probe is flattened to be substantially at right angles to the arms of the U.

References Cited in the file of this patent UNITED STATES PATENTS 2,611,102 Bohlke Sept. 16, 1952 2,63 ,130 Hull Mar. 17, 1953 2,636,948 Pierce Apr. 28, 1953 2,730,649 Dewey Jan. 10, 1956 2,758,244 Dodds Aug. 7, 1956 2,773,213 Dodds Dec. 4, 1956 

